Farmers and ranchers will for the first time need a prescription from a veterinarian before using antibiotics in farm animals, in hopes that more judicious use of the drugs will reduce the human deaths that result each year from the drugs’ overuse.

The Food and Drug Administration (FDA) announced the new voluntary rules Wednesday after trying for more than 35 years to stop farmers and ranchers from feeding antibiotics to cattle, pigs, chickens and other animals simply to help the animals grow larger. Using small amounts of antibiotics over long periods leads to the growth of bacteria that are resistant to the drugs’ effects, endangering humans who become infected but cannot be treated with routine antibiotic therapy.

Initially, the FDA is asking drugmakers to voluntarily change their labels to require a prescription; federal officials said drugmakers had largely agreed to the change. If some fail to impose the restrictions, the agency will consider a more forceful ban, said Michael Taylor, the FDA’s deputy commissioner for food.

The reason for the reliance on voluntary efforts is that the FDA’s process for revoking approved drug uses is lengthy and cumbersome, officials said. The last time the FDA banned an agricultural use of an antibiotic against the wishes of its maker, legal appeals took five years. In this case, hundreds of drugs are involved, each with myriad-approved uses in various animals.

At least 2 million people are sickened and an estimated 99,000 die every year from hospital-acquired infections, most of which result from resistant strains. It is unknown how many of these illnesses and deaths result from agricultural uses of antibiotics, but 80 percent of antibiotics sold in the United States are used in animals.

Taylor predicted the new restrictions would save lives because farmers would have to convince a veterinarian their animals are either sick or at risk of getting a specific illness. Just using the drugs for growth will be disallowed and, it is hoped, this will cut their use sharply. The new requirements will also make obtaining antibiotics more cumbersome and expensive.

How broadly farmers use antibiotics simply to promote animal growth is unknown.

The FDA believes veterinarians will be far less likely to endorse indiscriminate drug uses. While doctors have the power to use drugs in ways not approved by the FDA, veterinarians are allowed to give a prescription for antibiotics in feed and water only if such uses are approved by the FDA.

Dr. Christine Hoang of the American Veterinary Medical Association said her organization supported the new rules, although she said some remote or small farmers might have trouble abiding by the rules because there are fewer than 10,000 large-animal veterinarians in the United States.

The new rules generated mixed reactions. Laura Rogers of the Pew Campaign on Human Health and Industrial Farming called the new rules “the most sweeping action the agency has undertaken in this area,” while Caroline Smith DeWaal of the Center for Science in the Public Interest criticized them as “tragically flawed” because they relied too much on voluntary industry efforts.

The Animal Health Institute, an association of animal-drug makers, welcomed the new rules. But R.C. Hunt, president of the National Pork Producers Council, said small farmers and ranchers would have a hard time following the new rules.

Last month, U.S. District Judge Theodore Katz of the Southern District of New York, ordered the FDA to begin the process to ban indiscriminate farm uses of penicillin and tetracycline because of dangers to humans.